xm 



PHYLUM CHORDATA 



195 



a house, in such a way that a small three-sided portion (6) of 

 each scale comes to lie immediately beneath the epidermis, while 

 the rest (a) is hidden beneath the scales immediately anterior to 

 it. Besides the scales, the fin-rays belong to the exoskeleton, but 

 will be most conveniently con- 

 sidered in connection with the 

 endoskeleton. 



Endoskeleton. The vertebral 

 column shows a great advance 

 on that of the two previous classes 

 in being thoroughly differentiated 

 into distinct bony vertebrce. It 

 is divisible into an anterior or 

 trunk region and a posterior 

 or caudal region, each containing 

 about twenty-eight vertebrae. 



A typical trunk vertebra con- 

 sists of a dice-box-shaped centrum 

 (Fig. 868, CN) with deeply 

 concave anterior and posterior 

 faces, and perforated in the 

 centre by a small hole. The 

 edges of the centra are united 

 by ligament and the biconvex 

 spaces between them are filled 

 by the remains of the noto- 

 chord ; there are also articula- 

 tions between the arches by 

 means of little bony processes, 

 the zygapophyses (N. ZYG., H. ZYG.). To the dorsal surface of 

 the centrum is attached, by ligaments in the anterior vertebrae, 

 by ankylosis or actual bony union in the posterior, a low neural 

 arch (N. A.), which consists in the anterior vertebrae of distinct 

 right and left moieties, and is continued above into a long, slender, 

 double neural spine (N. SP.), directed upwards and backwards. 

 To the ventro-lateral region of the vertebra are attached by ligament 

 a pair of long, slender pleural ribs (R.) with dilated heads, which 

 curve downwards and backwards between the muscles and the 

 peritoneum, thus encircling the abdominal cavity. In the first 

 two vertebrae they are attached directly to the centrum, in the 

 rest to short downwardly directed bones, the parapophyses (PA. PH.), 

 immovably articulated by broad surfaces to the centrum. At the 

 junction of the neural arch with the centrum are attached, also by 

 fibrous union, a pair of delicate inter-muscular bones (I. M. B.), 

 which extend outwards and backwards in the fibrous septa between 

 the myomeres. The first and second abdominal vertebrae bear no 

 ribs. In the last three the neural spines (B, N. SP.) are single. 



FIG. 868. Salmo fario. A, one of the 

 anterior, and B, one of the posterior 

 trunk vertebrae ; C, one of the anterior, 

 and D, one of the posterior caudal verte- 

 brae. CUT. centrum ; 1MB. intermuscular 

 bone ; HA. haemal arch ; H. SP. haemal 



re ; H. ZYG. haemal zygapophysis ; 

 A. neural arch; N". SP. neural spine ; 

 N. ZYG. neural zygapophysis ; PA. PH. 

 parapophysis ; B. pleural rib. 



